HARQ is a key technique in next generation wireless systems that spans both MAC and PHY layers, and exploits time/frequency diversity and coding gain. In the HARQ scheme, incorrectly received coded data blocks are stored at the receiver rather than discarded, and when the retransmitted block is received, the two blocks are combined. While it is possible that when independently decoded, two given transmissions cannot be decoded error-free, it may happen that the combination of all the erroneously received transmissions gives enough information to correctly decode the block.
There are two main ways of re-combining in HARQ:                Chase combining: every retransmission contains the same information (data and parity bits) and contributes more signal power;        Incremental redundancy: every retransmission contains some different information than the previous one. At every retransmission the receiver gains knowledge of extra information.        
Modern Forward Error Correction (FEC) decoders process probabilistic information in the form of Log-Likelihood Ratios (LLR), also known as soft bits. The LLR values are the logarithm of the ratio of the probability of a certain bit being a ‘1’ to that of it being a ‘0’, ln [p(1)/p(0)]. When a soft bit (LLR) equals ‘0’, which is ln(1), it is equivalent to no or little information about the bit, which is equally likely to be a ‘0’ or a ‘1’ (i.e., p(1)=p(0)). When a soft bit (LLR)→∞ (or strongest or full-scale positive number in a fixed point representation), it means the bit is more likely to be a ‘1’, while an LLR→∞ (or strongest or full-scale negative number in a fixed point representation) means the bit is more likely to be a ‘0’. In practice, instead of infinity, the range is a fixed range. For example, an LLR value may be an integer between −128 and +127, which can be represented by 8 binary bits.
Traditionally, the incorrectly decoded retransmissions are stored and combined in the receiver in the form of Log-Likelihood Ratio (LLR) values derived from the input of the Forward Error Correction (FEC) decoder (so-called a-priori information). The FEC decoder essentially exploits the redundant information of the codeword to strengthen the input (a-priori) LLR values of the encoded bits until LLR values are reached from which each bit's value can be concluded.
FIG. 1 is a block diagram illustrating a prior art receiver having a HARQ combining module and a FEC decoder. Receiver 10 includes a demodulator 12 for demodulation of a carrier wave and for outputting soft symbols. During the demodulation process in demodulator 20, channel estimation is performed and, accordingly, channel correction is applied to the samples, so as to output improved samples. In addition, during channel estimation and correction, Carrier Frequency Offset (CFO) and Sampling Timing Offset (STO) are estimated and the received samples are corrected, accordingly, and output as soft symbols.
Receiver 10 further includes a LLR calculator and HARQ combining module 14, which is coupled to demodulator 12, for computing the LLR values (soft bits) of each soft symbol, and for combining the LLR values of the current transmission with LLR values of previous transmissions of unsuccessfully decoded code words. Receiver 10 further includes a HARQ memory 18, which is coupled to LLR calculator/HARQ combining module 14, for storing the a-priori LLR values from the LLR calculator/HARQ combining module 14. The LLR values of each transmission stored in HARQ memory 18 are input into LLR calculator/HARQ combining module 14 for combining with LLR values of the following retransmission of the un-decoded data. LLR calculator/HARQ combining module 14 is further coupled to a FEC decoder 16, for decoding the soft bits and for outputting decoded hard bits.
Recently, different schemes of turbo-equalization (TEq) and Parallel and Successive Interference Cancellation (PIC or SIC, respectively) became a baseline for advanced multi-stream and multi-user receivers. These techniques exploit the a-posteriori information available on the output of the FEC decoder to remodulate each interfering stream individually, then subtract it from the received signal, thus improving the signal-to-interference ratio of the other streams and enabling their decoding. This process is done iteratively, either in parallel on every stream (PIC) or stream-by-stream (SIC), until all the streams are decoded or a maximum number of allowed iterations is achieved (in which case a HARQ retransmission will be requested).
This a-posteriori information is commonly an improved version of the input LLR values enriched with additional (so called extrinsic) information, related to the probability of each decoded bit. However, this additional information is discarded and thus, the benefits of the additional performance gain brought by the FEC decoder are not exploited, especially not in the case of HARQ. In addition, in the vast majority of commercially available decoders, calculation of a-posterior soft bits is not required, hence not output. Usually one seeks only the decoded hard bits (the final output of the decoder) of the payload.
One of the drawbacks of HARQ is a decrease in link capacity, since several retransmissions of the same information occur at the expense of new information that otherwise could be transmitted. Accordingly, any improvement in HARQ performance will effectively lead to a decrease in the required number of retransmissions and, thus, to an increase of the link capacity and data rates.